1. Field of the Invention
The present invention relates to a circuit board and a method for manufacturing the same. In particular, the present invention relates to a circuit board using a reinforcer sheet with density variations in its in-plane direction and a method for manufacturing the same.
2. Description of Related Art
In recent years, accompanying the reduction in size and weight and the improvement in function and performance of electronic equipment, there has been an increasing demand for low-cost multilayered circuit boards allowing a high density mounting of semiconductor chips such as large-scale integrated circuits (LSIs), in the field of not only industrial appliances but also home electronic appliances.
In order to respond to such a market demand, the technology has been developed in which, instead of a conventional ceramic multilayered board, a resin multilayered circuit board that can be supplied at lower cost is made suitable for the high density mounting (a high density wiring board).
An example of such a circuit board includes a resin multilayered board having an inner-via-hole structure over all the layers as disclosed in JP 6(1994)-268345 A. This resin multilayered board adopts an inner via connection method that can connect desired positions of desired wiring layers by an electrically conductive paste, namely, the inner-via-hole structure over all the layers, thereby providing a low cost circuit board suitable for the high density mounting.
In a method for manufacturing this circuit board, inner via holes first are formed in a compressible insulator layer (an aramid-epoxy prepreg), and an electrically conductive paste is filled in the through holes. Thereafter, copper foils are superposed on both sides of the insulator layer, followed by heating and compression with a hot press, thereby curing resins in the insulator layer and the electrically conductive paste. This adheres the copper foils to the insulator layer and electrically connects the copper foils on both sides via the electrically conductive paste. Finally, the copper foils on both sides are processed into a wiring pattern, thus completing a double-sided circuit board.
Because of its high-density wiring and low connection resistance with less variation, this circuit board is highly valued in the market.
The reason why the high-density wiring is needed has been described above, while the usefulness of the connection resistance with less variation will be described in the following. That is, circuit resistance including the connection resistance is an important parameter for a circuit design. Accordingly, if the circuit resistance varies from one product to another, this causes a problem in that the circuit design is impossible or that the circuit resistance of the product deviates from a designed value so that the product cannot operate properly. Thus, the connection resistance has to have less variation.
Especially, more inner via holes are involved in one circuit in the connection by the inner via holes than in the conventional connection by the through holes. Therefore, there is a more stringent requirement with respect to variations.
However, the above-described circuit board technology having the inner-via-hole structure over all the layers has had a following problem. As the insulator layer mentioned above, a composite material of an aramid non-woven fabric as a reinforcer and an epoxy resin (an aramid-epoxy base material) is used. In this case, because of their high moisture absorbency, it is necessary to manage aramid fibers so as to prevent them from absorbing moisture by vacuum-packing or the like. Such management would increase cost.
On the other hand, a glass-epoxy base material used in a general circuit board is a material obtained by impregnating woven fabric made of glass fibers with an epoxy resin. Since the glass fibers do not absorb moisture, using the glass-epoxy base material is advantageous in the management of water absorption. Furthermore, because its mechanical strength is high, it has been desired that the glass-epoxy base material is used as the insulator layer so as to achieve the circuit board having the inner-via-hole structure over all the layers by the inner via connection.
However, when simply attempting to apply the above-described inner-via-hole technology over all the layers to the glass-epoxy base material, there arises a problem in that the variations in the connection resistance of the inner via holes increase. The inventors of the present invention conducted a study to find as its cause that the glass woven fabric serving as the reinforcer had variations in density (portions in which warps and wefts overlap each other and those in which they do not) in an in-plane direction. More specifically, in the hot press process of heating and compression, the inner via holes provided in the low-density portion of the reinforcer (where the warps and wefts do not overlap each other) expand laterally because there is less reinforcement on their side wall surfaces. In other words, applied pressure dissipates laterally. Consequently, a sufficient compression force is not applied in the longitudinal direction of the inner via hole, so that electrical conductors cannot be connected sufficiently, thus increasing an electrical connection resistance.
The unevenness in thickness and density in the in-plane direction causes such variations in the electrical connection resistance not only in the glass cloth impregnated with an epoxy resin but also in non-woven fabric, a sheet and a film.